Laser systems including fiber amplifiers are commonly used in many applications, including telecommunications applications and high power military and industrial fiber optic applications. For example, both U.S. Pat. No. 5,946,130, issued Aug. 31, 1999 to Rice and U.S. Pat. No. 5,694,408 issued Dec. 2, 1997 to Bott et al. describe many such applications in which laser systems including fiber amplifiers are employed including the processing of materials, laser weapon and laser ranging systems, and a variety of medical and other applications. In this regard, such fiber amplifiers generally include optical fibers that passively transmit optical power, fibers that experience or are designed to enhance performance of a laser through nonlinear optical processes such as Raman-shifting and Brillouin scattering, as well as optical fibers that are doped with a lasing ion embedded in the fiber material. For more details of such applications, see U.S. Pat. No. 5,832,006 issued Nov. 3, 1998 to Rice et al., the contents of which are hereby incorporated by reference in its entirety, which describes coherently phasing Raman fibers in a high brightness array. In addition, see U.S. Pat. No. 6,363,087 issued Mar. 26, 2002 to Rice, the contents of which are also incorporated by reference, which describes a multimode Raman fiber amplifier.
Optical fiber amplifiers are designed to increase the power output levels of the signals propagating therealong. One conventional optical fiber amplifier design is an end-pumped dual-clad fiber, such as that described in U.S. Pat. No. 4,815,079 issued Mar. 21, 1989 to Snitzer et al. Referring to FIGS. 1A and 1B, the dual-clad fiber 1 has a single mode signal core 2, a multi-mode pump core 3 surrounding the signal core, and an outer cladding layer 4 surrounding the pump core for confining pump energy within the pump core such that signals propagating through the signal core are amplified. The signal core will typically be doped with one or more rare earth elements such as, for example, ytterbium, neodymium, praseodymium, erbium, holmium or thulium.
In operation, pump energy is coupled into the pump core 3 at the input end 5 of the fiber. The pump energy then propagates through the pump core until it is absorbed by the dopant in the signal core 2, thus amplifying signals propagating through the signal core. Although dual-clad fibers 1 can have different sizes, one typical dual-clad fiber includes a signal core that has a diameter of 8-10 μm and a pump core that has cross-sectional dimensions of 100-300 μm. End-pumped dual-clad fiber amplifiers of this size can typically reach fiber energy power levels of 100 W. Recent demonstrations of fiber lasers operating with output powers in excess of one kilowatt have been performed. The use of large core multimode fiber designs have increased the power output beyond those limits encountered in single-mode fiber lasers. The large core multimode fibers can have a doped core with cross-sectional dimensions on the order of 30 to 50-microns in diameter or larger. The pump cladding sizes have increased to about 350 microns in diameter with an outer cladding to over 500 microns in diameter. Additionally, progress has been made on construction of photonic, bandgap or “holey” fibers that have voids within the fiber to improve the wave-guiding properties of the fiber construction. All of these designs have a need to remove excess heat from the doped core of the fiber laser oscillator or amplifier.
In general, laser systems can scale arrays of fiber amplifiers to produce higher power by coupling the output energy from a bundle of relatively low-power, fiber amplifiers. It will be appreciated, then, that scaling fiber amplifiers is generally driven by the ability to coherently combine the output of multiple fiber amplifiers. In this regard, to combine the outputs, the individual fiber amplifiers must typically comprise low-noise, single-mode amplifiers that polarize the output energy such that the energy from the individual amplifiers can be efficiently combined into an integrated array. Stable control of the thermal environment of the fiber amplifier is required to maintain the low-noise operation for phasing of fiber arrays. Uncontrolled heating of a fiber creates significant changes in the optical length and hence the optical phase of the signal at the output ends. Rapid heating would cause significant tracking issues for phase modulation and control techniques needed to coherently phase the outputs of two or more fibers.
Although laser systems generate coherent output power in a manner that is intrinsically efficient, a physical limit referred to as the quantum defect limit typically limits the energy conversion process of such systems. As is known, quantum defect is the difference in the photon energy at which the process is pumped versus the energy of the radiated “lasing” photons. For the most efficient systems, 70% to 90% of the pump energy from laser diode pump photons is converted to output energy. For less efficient arrangements, however, the efficiencies can lie in the range of 40% to 50% or lower. Generally, the quantum defect limit, as well as spontaneous radiation losses, miscellaneous optical absorption losses and other non-productive processes, lead to thermal energy release that heats the fiber amplifier.
In continuous or quasi-steady operational modes, the temperature of the fiber amplifier will rise until an equilibrium heat transfer condition is established. For high power laser systems, such a rise in temperature can result in elevated temperatures in the core of the fiber amplifier (typically where doped lasing media is located in fiber amplifiers having a doped core). In turn, the elevated core temperature can degrade the efficiency of the laser system, lead to unacceptable optical distortions or, in the extreme, to failure of the fiber amplifiers or surrounding system materials and components. Thus, it would be desirable to design a fiber amplifier and optical fiber laser system that conduct heat away from the core of the fiber amplifier or otherwise decrease the amount of heat generated in the fiber core.